100 ohm heater resistors fried

[Toobgeek]

I've got a Deluxe Reverb clone that I built about 8 years ago.  I was playing it today and there was the burning electronic smell.  I got the chassis out the amps as quickly as I could and found the two 1/2 watt 100 ohm resistors were burned.  What is the best way to track down the cause?  I don't know what my next move is.  Thanks for any help you can offer!!

[sluckey]

Check for dc voltage on the tube filaments. Got any?

[Toobgeek]

I pulled all the tubes and got 6.7 vac on standby.

[sluckey]

I pulled all the tubes and got 6.7 vac on standby.

Look for a short between pins 2 and 3 of both output tube sockets. Look for a short between pins 6 and 7 of both output tube sockets. Fix any visible shorts. Also look for carbon traces between pins 2 and 3 and also between pins 6 and 7 of both sockets. Replace sockets if you see any carbon traces.

Next unsolder one end of each 100Ω resistor and check for dc voltage on the tube filaments. Cannot be on standby! If no dc voltage is present, plug in a 6V6 tube and check for dc on the filaments. If no dc voltage, plug in the other 6V6 and check for dc voltage on the filaments.

I suspect a short from B+ to the filaments, probably on the output tube sockets. Could be an internal tube short, or maybe on the socket between pins 2 and 3, or 6 and 7 if the 470Ω screen resistors are mounted on the sockets.

6.3VAC should not harm those 100Ω resistors but sometimes **it happens. If you don't find any dc voltage on the filaments, install new 100Ω resistors and rock on.

[EL34]

I have seen that happen when the very large voltage on the plates of the power tubes (pin 3) arcs across the tube sockets to pin 2
That is almost a pure path to ground

All it takes is some dust to bridge the gap a little bit or someone sprays tuner cleaner on the power tube sockets (bad Idea)
Or some other substance that is conductive gets on the power tube sockets

Make sure you pull the power tubes and look very closely for carbon arc paths.
Front side and back side of the sockets
It could have arc'd across the bottom of the tube base itself but it still leaves carbon film

Any small traces of carbon will short out again
I have seen plastic tube sockets that arc'd so deep you had to replace them

Good thing you had 100 ohm resistors which are cheap to replace
I have seen that happen and fry the heater winding on amps that have a heater center tap

[eleventeen]

I'll just bring this up as a very small possibility, and maybe it is only theoretical, since you said you first encountered this when the amp was "working".


But "working" could mean one of two things: 1: You were actually playing thru the amp, touched utterly nothing, and in between playing two chords or two notes, suddenly smoke/smell appeared. 2: It could also mean the amp was working fine, you decided to look at your 6V6 tubes, turned power off, pulled the tubes, then put them back in, even putting them back in the same sockets they came out of. Nothing changed, right?


But I have seen this happen when the output tube sockets are very worn and one (or, I suppose both) of the output tubes has been inserted one pin off because the keyway in one or the other sockets no longer uniquely locates the 6V6 in its socket. I could hardly believe it when I saw it, because I saw it in a school amp I was fixing, and I can't imagine the 6V6 were pulled out more than once or twice in the entire life of the amp (an 80's Deluxe Reverb II) So how could the 6V6 sockets be so worn that you could put the tube in one or more pins off? 


Well, they were, whether I believed it or not. Did I look to see whether the 6V6 tubes were installed correctly? No, I just placed the tube over the socket and spun it until (I thought) the key on the base found the notch and the tube dropped into index, like I've done 40,000 times over 40+ years of working on amps. The key on the base locates them so they can only be installed one way right? Wrong! I was able to install the 6V6's 3 or 4 different ways/rotations. I could hardly believe it. It could *also* be a situation where the socket is slightly worn and we go to install a JJ 6V6 or some other "modern" brand of 6V6 and center keyed post is microscopically smaller than a 1960's RCA/GE 6V6 for reasons we can only speculate about.


When Doug said "I have seen that happen when the very large voltage on the plates of the power tubes (pin 3) arcs across the tube sockets to pin 2....That is almost a pure path to ground"
That is what will happen should the tube be installed rotated one pin off. And obviously, if the center key goodie is broken off and the tube is reused anyway, it would be easy as pie to get the tube rotated one or more pins off.


So as I said, it's a really long shot, but it is less of a long shot if you never even suspect it might be a possibility.


It is also true that this is the situation where you are really happy that you have the "synthetic" center tap versus the "actual" center tap on the 6.3 winding.

[Toobgeek]

One of the 6V6 tube sockets is burned on pins 2 and 3.  I don't see any evidence of arcing though (and I was looking through a magnifier).  I guess the safe money is to replace that tube socket and the tubes?

Thanks guys..I REALLY appreciate the help!

[Toobgeek]

I replaced the sockets and the tubes.  Once I got the socket out of the amp...it had arced.  The amp sounds great....the new tubes didn't hurt the tone~   

Thanks again!

[sluckey]

I replaced the sockets and the tubes.  Once I got the socket out of the amp...it had arced.  The amp sounds great....the new tubes didn't hurt the tone

A very real example that will arc the tube socket between pins 2 and 3 is to try to play the amp without a speaker connected. The open circuit on the OT secondary will kick back a very high voltage spike on the primary which is connected to the tube's plate. This high voltage spike will try to arc to ground someway. Often times that path is thru the filament pin. The resulting carbon (burn) trace will usually pop those 100Ω resistors, or even worse if the filament uses a real filament center tap.

Not saying this was your case, but just be aware of the dangers of playing a tube amp without a speaker.

[HotBluePlates]

One of the 6V6 tube sockets is burned on pins 2 and 3.  I don't see any evidence of arcing though ...

The visible burn path between the pins is the evidence of arcing.

[EL34]

I replaced the sockets and the tubes.  Once I got the socket out of the amp...it had arced.  The amp sounds great....the new tubes didn't hurt the tone~   

Thanks again!

Ta Da